The invention relates to an apparatus for the continuous tight heat-sealing of the longitudinal overlapping edges of tubular pieces generally of thermoplastic film, the thickness of which may be limited, the apparatus being particularly suitable for machines for packaging products in barrier-effect stretch film, as disclosed in patent application No. GE2001A-96 and No. BO2002A-410, the property of the present applicant, which are referred to in full.
In this type of machine, the technical problem that had to be solved was to do with the continuous tight longitudinal sealing of the overlapping edges of the packaging film as it passes over the tubularizing mandrel. It has been impossible to find, either on the market or in the known literature, an apparatus capable of solving this technical problem. It should be remembered that the film used is characterized by high grip, high extensibility and limited thickness, of the order of tens of microns, e.g. about 40 microns. The seal must be formed and interrupted rapidly, in a few hundredths of a second, on a film which begins with a standing start, advances rapidly at speeds of for example more than 0.6 metres per second, and then stops again. The seal to be formed on the longitudinal edges of the film must be extremely reliable, absolutely tight, must be elastic so as to react without defects to the stretching applied to the film, must not form hard beads that could damage packagings with which the seal comes into contact when the packagings are stacked, and must also be visually pleasing.